Limiter for radio receiving apparatus



June 8, 1948. SHEA 2,443,057

LIMITER FOR RADIO RECEIVING APPARATUS Filed April 12, 1944 3 3 DlSgIFlJkiATOR RF AMPLIFIER AMPUHER MIXER SIGNAL IF AMPLIFIER REPRODUCER PLATE CURRENT NEGATIVE CONTACT BIAS POTENTIAL GRID VOLTAGE Inventor: Richard F. Shea,

His Attorney Patented June 8, 1948 LIMITER FOR RADIO RECEIVING APPARATUS Richard F. .Shea, Fairficld, Conn., assignor to General Electric Company, a corporation of New York Application April 12, 1944, Serial No.'5 30,671

6 Claims. (01. 178- 44) My invention relates to amplitude limiting means for electric signal waves, and is particularly applicable to radio receiving apparatus of the frequency modulation type.

It has for one of its objects to provide new and improved, and particularly simple and efficient, means for limiting the amplitude of a frequency modulated signal wave.

It is a further object of my invention to provide new and improved means for limiting the amplitude of a signal wave of relatively low voltage. I

'More specifically, it is an object of my invention-to provide an amplitude limiter of the electron discharge device saturation type arranged to maintain a signal wave within predetermined limits of amplitude independentlyof' the cutoff voltage of the discharge device.

Briefly, the above and other objects of my invention are attained by operating an electron discharge device at only very slight negative grid potential, for example, the grid contact potential, and supplying to the grid onlypositive half cycles of a signal wave. Thus, the electron discharge device is non-conductive during'negative half cycles and limits the amplitude of positive half cycles by plate current saturation.

My invention will be more fully understood and its objects and advantages further appreciated by referring now to the following detailed specification taken in conjunction with the accompanying drawings in which Fig. 1 is a schematic circuit diagram, partially in block form, of a radio receiving apparatus embodying my invention, and Fig. 2 is a, graphical representation of certain of the characteristics of the apparatus shown at Fig. 1.

Referring now to the drawings, and particularly to Fig. 1, I have shown my invention by way of example as applied to a frequency modulationradio receiving apparatus of the superheterodyne type. While my invention is particularly applicable to such apparatus, I wish to have it understood that it is in no way limited thereto. but is useful wherever it is desired to limit the amplitude of a signal or carrier wave to a predetermined maximum value.

The apparatus shown at Fig. 1 comprises an antenna. Ill for receiving a frequency modulated signal wave and supplying the wave to a signal channel including a suitable number of stages of radio frequency amplification, a converter or mixer for heterodyning the received wave with local oscillations of different frequency to derive asignal wave of suitable intermediate frequency,

and a suitable number of stages of intermediate frequency amplification. The apparatus so far described forms no part'of my present invention and is indicated in block form only and identified by the reference numeral II. From thelast intermediate frequency amplifierthe signal wave is supplied to a transformer 12 tuned to the intermediate frequency and having the mid-point of its secon-dary winding l3 connected to ground. The positive half cycles of the signal wave appearing across one half of the transformer secondary winding l3 are supplied through a diode rectifier ill from one terminal of the winding l3 to a load resistor l5 having one end connected to gr0und.1 The diode rectifier I4 is a unilateral conducting device of the electron discharge type having a plate-to-cathode capacitance which may be represented by a small capacitor 1 6 shown in broken lines and connected between the anode and cathode of the rectifier 14. For the purpose of neutralizing the effect of the capacitor [6 upon negative half cycles of the signalwave, a small capacitor ll having a capacitance substantially equal to the interelectrode capacitance I6 is connected between the other terminal of the transformer secondary winding l3 and the positive terminal of the load resistor I5.

The positive half cycles of the signal wavev appearing across the load resistor [5 are supplied through a resistor [8 to the control electrode [9 of an electron discharge device having a cathode 2I. a screen electrode 22, a suppressor electrode 23, and an anode 24. The cathode 2| is con nected to ground, and the anode 24 of the dis charge device 20 is connected through a tuned output circuit 25 and a resistor 26 to a suitable source of positive unidirectional current supply indicated on the drawings byB+. Intermediate frequency currents appearing in the output circult of the electron discharge device 20 are bypassed around the unidirectional current supply source through'a capacitor 21. The suppressor electrode 23 is directly connected to' the cathode 21 in well-known manner, and the screen electrode 22 is connected through a resistor 28 to the positive terminal of the unidirectional current supply source 13+. A by-pass capacitor 29 prevents intermediate frequency currents appearing upon the screen electrode from passing through the unidirectional current supply source.

The tuned output circuit 25 includes the pri.

mary winding 30 of a transformer 3!, from the secondary winding 32 of which the frequency modulated signal wave of limited amplitude is supplied to a frequency modulation discriminator.

a suitable number of stages of signal frequency amplification, and a suitable signal reproducing device. This latter apparatus forms no part of my present invention and has been shown in block form only and identified by the reference numeral 33. It will be understood that the signal reproducing apparatus may be a loudspeaker, headphones, or the like for receiving audible signals or, as in the case of a television receiver, the signal reproducing device may be a cathode ray tube or the like.

It will be noted that the cathode 2| of the electron discharge device 20 is connected to the control electrode l9 only through the load re sistor l and the resistor I8. With this arrangement the control electrode I9 is biased only slightly negative with respect to the cathode by the col lection of random electrons from the discharge stream upon the control electrode. This slight negative or contact potential is maintained upon the control electrode through the resistors l5 and 18. Accordingly, the discharge device 2| is operated in the region of plate current saturation and considerably above cutoff.

The operation of my new and improved wave limiting apparatus will be more fully understood by referring now to the diagram of Fig. 2 taken in connection with Fig. 1. At Fig. 2, I have shown the plate current-grid voltage characteristic of a typical electron discharge device of the type of the discharge device 20. It is characteristic of such a device that, with the control electrode unbiased, or biased only slightly negatively with respect to the cathode, the plate current is only slightly below its saturation value. I have indicated at Fig. 2 by a line 35 the slightly negative contact potential to which the control electrode is of the discharge device 2!! is biased in operation. The curve 36 of Fig. 2 represents a typical signal or carrier wave having undesired variations in amplitude and appearing across one half of the transformer secondary winding i3. From Fig. 1, it may be observed that only the positive half cycles of the wave 36 are supplied to the control electrode l9 by conduction through the diode rectifier I4. These positive half cycles produce in the output circuit oscillations represented by the curve 31 of Fig. 2. The negative half cycles of the wave 38 at the transformer l3 do not appear across the load resistor l5 and are not supplied to the control electrode is of the discharge device 20, so that the discharge device 20 remains non-conducting during the negative half cycle period.

It will be evident from Figs. 1 and 2 that negative voltages appearing at the upper terminal of the transformer winding l3. while not supplied by conduction through the rectifier it, might be supplied by voltage division between the diode capacitance I6 and the control electrode-to-cathode capacitance of the discharge device 20. It is for the purpose of neutralizing such negative voltages that the capacitor I1 is connected between the lower terminal of the transformer winding l3 and the positive terminal of the load resistor l5. It will be clear that during negative half cycles or the upper terminal of the transformer winding l3, the lower terminal of the winding 13 is positive, and that such positive voltage is supplied through the capacitor H to the positive terminal oi the load resistor l5. Accordingly, during negative half cycles of the upper terminal of the winding l3, any negative voltage which may be impressed upon the upper terminal of the load resistor 15 through the capacitance of the diode rectifier I4 is neutralized by an equal positive voltage impressed upon the same terminal of the load resistor through the neutralizing capacitor IT.

From the foregoing description, it will now be understood that my invention provides means for limiting the amplitude of signa1 or carrier waves of very low intensity. Heretofore, saturation limiters have functioned by tube cutoff to limit the negative half cycles of the wave so that it has been necessary to provide a grid voltage swing between cutoif and saturation. By my invention, all negative half cycles are entirely eliminated, so that only single-ended amplitude limitation is necessary.

While I have illustrated only a preferred embodiment of m invention by way of illustration, many modifications will occur to those skilled in the art and I, therefore, wish to have it understood that I intend in the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a source of signal waves having undesired variations in amplitude, an electron discharge device having an anode, a cathode and a control electrode, means including a unilateral conducting device for supplying to said control electrode only positive half cycles of waves from said source, and means for maintaining the unidirectional potential of said control electrode nearly equal to the potential of said cathode.

2. In combination, a source of signal waves having undesired variations in amplitude, an electron discharge device having an anode, a cathode and a. control electrode, means including a unilateral conducting device for supplying to said control electrode only positive half cycles of waves from said source, and means for maintaining said control electrode at a small negative bias potential with respect to said cathode thereby to limit the amplitude of said positive half cycles by plate current saturation.

3. In combination, a source of signal waves having undesired variations in amplitude, an electron discharge device having an anode, a cathode and a control electrode, means including a unilateral conducting device coupling said control electrode to said source and arranged to supply to said control electrode positive half cycles only of waves from said source, means for maintaining said control electrode at a small negative unidirectional potential with respect to said cathode, and capacitive means coupling said control electrode to said source in opposite phase relation with respect to said unilateral conductive coupling thereby to neutralize the capacitance of said unilateral conducting device.

4. A frequency modulation receiving apparatus including a. channel for transmitting signal waves modulated in frequency in accordance with a desired signal and having undesired variations in amplitude, said channel including a transformer having its mid-point grounded, a load resistor having one end connected to ground, unilateral conducting means for connecting one terminal of said transformer to the other end of said resistor thereby to supply positive half cycles of said signal waves across said resistor, a capacitor connecting the other terminal of said transformer to said other end of said resistor thereby to neutralize the capacitance of said unilateral conducting means during negative half cycles of said waves, and an electron discharge device having an input circuit connected across said resistor and arranged to limit the amplitude of said positive half cycles by plate current saturation.

5. A frequency modulation receiving apparatus including a, channel for transmitting signal waves modulated in frequency in accordance with a desired signal and having undesired variations in amplitude, said channel including a transformer having its mid-point grounded, a load resistor having one end connected to ground. means including a unilateral conducting device for connecting one terminal of said transformer to the other end of said resistor, said means being arranged to supply positive half cycles of said Waves across said resistor, capacitive means including said unilateral conducting device and connecting both terminals of said transformer to said other end of said resistor thereby to neutralize negative half cycles of said waves, an electron discharge device having an anode, control electrode and a grounded cathode, means for maintaining said control electrode at a small negativebias P tential with respect to said cathode, and means for supplying said positive half cycles of said signal waves appearing across said resistor to said control electrode thereby to limit the amplitude of said positive half cycles by plate current saturation.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,182,322 Roberts Dec. 5, 1939 2,207,587 Kaar July 9, 1940 2,271,203 Okrent Jan. 27, 1942 2,295,323 Armstrong Sept. 8, 1942 2,353,018 Duke Oct. 24, 1942 

